It’s never quite that simple…

Mike and I had a long weekend break to visit family and friends in the South Island at the end of August. In accordance with trying to minimise the footprint of our travel (see our strategy here), we booked tickets for the ferry from Wellington to Picton and then hired a car from Picton rather than hopping on a plane.

Just now, I checked how the figures stack up regarding how much CO2e we roughly saved… the results were not as starkly different as I would have expected, showing these things are never that simple!

For a start, it is difficult finding figures that give you an accurate idea of emissions for ferries. Referring to DEFRA’s emissions conversion factors, a foot passenger on a ferry apparently uses 0.019kg CO2e per km (which would equate to 7.8kg CO2e for both of us, both ways). However, when you look at the methodology behind this figure it is calculated only for mixed passenger and freight ferries, not for passenger only ferries, which would have a higher factor. The Interislander ferry we took is predominantly a passenger ferry (to be confirmed), so I also found this higher estimate, which would put our ferry journey at 18.4kg CO2e for a return trip for two of us.

With cars I’m still not totally sure what assumptions are made with the various calculators out there – on this occasion I used as I could enter the fuel efficiency alongside fuel type and distance travelled. So the car distance travelled from Picton to Christchurch came out at 181-206kg CO2e (for the return journey of 676km travelled, with a fuel efficiency of 6-7 litres per 100km).

This means a return journey by ferry from Wellington to Picton and car from Picton to Christchurch for two of us is 188.8-224.4kg CO2e, accounting for the variances in the estimates above.

For flights, we have previously done lots of research into flight emissions calculators and found the greatest rigour and transparency in Atmosfair’s emissions calculator. For two people taking a return flight from Wellington to Christchurch, this would be 216-271kg CO2e, depending on the aeroplane used).

Although the median value is lower for the car-ferry option compared to the plane, it shows that depending on the specific factors of the journey we took, taking a ferry and car could have had a higher footprint than flying, which I find quite shocking. I am surprised there is not a greater disparity between them, and it shows the assumptions in our carbon offsetting strategy are not absolutely steadfast.

My conclusions from this exercise:

Flying makes it easy, fast and cheap to travel long distances which is what leads to it being so impactful in terms of emissions – you can do many more long journeys as a result. Depending on the context, flying can be an efficient way to travel, it just leads to Jevons paradox i.e. it encourages more miles to be travelled thus undermining the (potential) efficiencies of the individual journey made.

This calculation shows how accurate estimates related to the specific context are really needed to have confidence in knowing for sure which one is better – given these close proximities in numbers.

In future I think we will always assume to travel in a way that avoids flying.  This means we have to take into account longer journey times and possibly more expense for going away, making it a less trivial decision to go. And then we will do the actual calculations to check if it really does result in lower carbon emissions to travel that way compared to flying…